1. Field of the Invention
The present invention relates to a scanner motor which is applicable, for example, to a driving mechanism of a rotary polygonal mirror for scanning a photosensitive body with a laser beam or the like in a recording apparatus, such as a laser beam printer.
2. Related Background Art
In recent years, a rotating apparatus, which achieves a high-speed or high-precision rotation for driving a rotary polygonal mirror, is needed in a laser beam printer and the like. Particularly, a bearing for achieving a non-contact rotation is used to obtain a high-precision deflective scanning apparatus.
FIG. 1 shows a scanner which uses a dynamic pressure fluid bearing, i.e., a kind of non-contact bearing, and which is usable in a deflective scanning apparatus, such as a laser beam printer. In the apparatus of FIG. 1, a rotary shaft 1 is rotatably fitted into a sleeve 2. To a lower portion of the sleeve 2, a thrust plate 3 is fixed together with a fixing plate 4, and thus the sleeve 2 is fixed to an outer cylinder 5 connected to fixing plate 4.
A flange 6 is fixed to the rotary shaft 1. A rotary polygonal mirror 7 is then fixed to an upper portion of the flange 6, and a yoke 9 with a driving magnet 8 fixed thereto is fixed to a lower portion of the flange 6. A stator 10 is fixed to the outer cylinder 5 in a state in which the stator 10 faces the driving magnet 8. Thus, a driving motor is constructed.
A shallow groove 11 is formed on a surface of the thrust plate 3 facing an end portion of the rotary shaft 1, and thus a dynamic pressure thrust bearing is built. Herringbone shallow grooves 14 and 14' are formed on two portions of an outer peripheral surface of the rotary shaft 1 facing an inner peripheral surface of the sleeve 2. Further, a spiral shallow groove 15 is formed on the outer peripheral surface of the rotary shaft 1 to facilitate the flow of lubricating fluid in the dynamic pressure thrust bearing. On the inner surface of the sleeve 2, a recess portion 16 is formed between the herringbone shallow groove 14 and the spiral shallow groove 15, and a small-diameter hole 17 is formed in the sleeve 2. Thus, the stability of the lubricating fluid flow is assured.
Further, a relief portion 18 is formed between the two herringbone shallow grooves 14 and 14', and another relief portion 19 is formed between the lower herringbone shallow groove 14' and the dynamic pressure thrust bearing portion. Thus, the reduction of the fluid bearing portion is minimized.
Recently, a bearing apparatus using a ceramic material and the like has been developed.
In such bearings, however, the following technical disadvantages exist which diminish the accomplishment of a high-speed and high-precision deflective scanning apparatus.
First, in dynamic pressure bearings using liquid, such as oil and grease as the fluid, the viscosity resistance of the fluid and, hence, the torque loss increases as the speed increases, and thus heat generation and power consumption increase.
Second, in dynamic pressure fluid bearings using gas, such as air, as the fluid, the bearing is vulnerable to the invasion of dust and moisture and, hence, it difficult to handle. Further, the so-called biting phenomenon is likely to occur when there is a contact between members due to vibrations and the like, when a high-speed rotation is conducted.
Third, in bearings using a ceramic material in a portion thereof, precision decreases when a change in temperature and the like occurs, even if a rotary polygonal mirror and so forth are initially assembled precisely, since the rates of thermal expansion of the different materials are substantially different from each other.